Aircraft canopies, and particularly stealth aircraft canopies, preferably include a low resistance (i.e., high electrical conductivity) layer (or layers) to prevent or reduce the buildup of static charge and to provide radar attenuation. Static charge can buildup on a canopy as the result of precipitation static and/or lightning strikes, and may interfere with various functions of the aircraft. By including a low resistance layer (or layers), an aircraft canopy can drain or dissipate static electricity and thereby prevent or reduce the buildup of static charge on the canopy. The low resistance layer (or layers) may be coated with a high resistance topcoat (e.g., a polyurethane antistatic coating), so long as the static charge can be transferred through the organic topcoat into the low resistance layer (or layers).
Modern jet aircraft canopies, such as F-22 stealth fighter canopies, are typically made of polymeric materials. Such materials are preferred because of their light weight, high strength, and ease of shaping. Most polymeric materials, however, do not meet the requirements for stealth aircraft, such as low sheet resistance and the ability to withstand extreme weather conditions. As a result, coatings (e.g., organic and inorganic coatings) are employed to impart high electrical conductivity and other necessary characteristics to the canopy.
Typically these coatings include a metal layer, e.g., a silver (Ag), platinum (Pt), palladium (Pd), or tungsten (W) layer, and anti-reflective metal oxide layers, e.g., indium tin oxide (ITO) or titanium dioxide (TiO2) layers, to impart electrical conductivity and transparency to the coating. Metal layers including silver have been preferred, as such layers exhibit high electrical conductivity and neutral color. However, silver lacks corrosion resistance, and the oxidation of silver to silver oxide reduces the flexibility and light transmission of the metal layer. Because silver is more susceptible to corrosion at higher temperatures, anti-reflective metal oxide coatings typically have been applied to the silver layer at reduced temperatures, which reduces the tensile elongation property of the resulting anti-reflective metallic oxide coating. Additionally, because environmental exposure (e.g., moisture, ultraviolet light, and/or acid rain) may result in the oxidation of silver, coatings including a silver layer often also include one or more organic layers, such as hydrophobic polymers, to protect the silver from environmental exposure. Although the organic layers may reduce the exposure of the silver to moisture, such coatings have exhibited limited service life due to the rapid degradation of the electrical and optical properties of the coating. Additionally, the metal oxide layers, for example the ITO layers, are typically limited to ultra thin layers, which limits the light transmittance of the coating.
Accordingly, a need still remains for electrically conductive coatings having greater durability and functionality.